Apparatus and method for generating repetitive pulses

Abstract: A modulator for producing repetitive short high voltage pulses includes a high voltage power supply (12), a plurality of pulse forming networks (40) each capable of being charged to a potential equal to twice the voltage of the high voltage power supply, and a plurality of switches (60) interconnecting the pulse forming networks. The switches have a first state connecting the pulse forming networks in parallel with the high voltage power supply to charge the pulse forming networks and a second state connecting the pulse forming networks in series to discharge the pulse forming networks into a load (70). The pulse forming networks (40) are charged by use of a resonant charging technique. The modulator further includes an input (22) configured to receive a triggering pulse, the triggering pulse causing the switches to change from the first state to the second state.

Description: The present invention relates to the generation of repetitive pulses of electrical energy and, more particularly, is applicable to a modulator to provide high voltage pulses with fast rise and fall times. Many types of electrical apparatus, such as microwave tubes used in radar transmitters and particle accelerators, require short rectangular high voltage pulses of electrical energy for their operation. These pulses may be of the order of tens or hundreds of kilovolts with a pulse width in the nanosecond or microsecond range. The repetition frequency of the pulses may range from one hundred to several thousand Hertz. The predominant technology for generating these pulses makes use of a pulse forming network which functions as an artificial transmission line. The network is charged to a desired voltage level and then is coupled to an external load by a switch. Since neither the network nor the switch can withstand the high voltage level required by the load, a transformer raises the network voltage to the required level. The switch is activated by external timing signals. This discharges the network into the primary windings of the transformer which provides the stepped up voltage pulse to the load. After each pulse has been transmitted, the charging of the network is repeated. The switch may be a solid state device or a gas filled tube such as a thyratron. A thyratron is filled with hydrogen gas and is capable of withstanding about 50 to 100 kilovolts between its cathode and anode. The thyratron has a grid disposed between the cathode and anode, and thyratrons of higher voltage ratings may have a plurality of such grids. Ionization of the hydrogen gas is initiated by a triggering voltage applied to the grid nearest the cathode, which overcomes the negative bias of the grid with respect to the cathode and initiates an arc discharge of current from the cathode to the anode. Thyratrons are generally preferred, as discussed for example in US-A-4375594, and are generally preferable to solid state devices, since they can withstand a much higher voltage in the "off" state and conduct higher current in the "on" state. US-A-3845322 shows an alternative using spark gaps which are switched by voltages tapped from series-connected resistors, whereas series-connected inductances are provided to connect capacitors in parallel. Another form of pulse generator is disclosed in US-A-3746881 also using spark gaps and capacitors. Inductances couple a triggering generator to spark gaps coupled to a stacked plurality of chargeable capacitors. It is known from US-A-4081719 negatively to bias the grid of a double-ended hydrogen thyratron of the tetrode type in order to maintain voltage hold-off and reduce dark current when the thyratron is in its nonconducting state. However, a drawback with the prior art pulse generators is that the transformer tends to degrade the quality of the pulse at high voltage levels and short pulse widths. When the transformer step-up ratio is high, the leakage inductance of the transformer is also high, which tends to increase the rise and fall times of short pulses. Thus, it is increasingly difficult to provide substantially rectangular high voltage pulses having very fast rise and fall times. In accordance with one aspect of this invention, there is provided an apparatus for producing repetitive voltage pulses comprising: a high voltage power supply for producing a charging voltage; a plurality of pulse forming networks for storing charge; a plurality of thyratrons, arranged between respective ones of said pulse forming networks and an external load, the thyratrons being switchable via respective switch control inputs between an OFF state in which the pulse forming networks are connected in parallel with the high voltage power supply, for resonantly charging the pulse forming networks, and an ON state in which the thyratrons provide a current path connecting the pulse forming networks in series, for discharging the pulse forming networks into an external load; and input means to receive a trigger pulse and coupled to the switching devices so that receipt of a trigger pulse by the input means will switch the thyratrons into the ON state; characterised by a biasing power supply for negatively biasing the thyratrons and a plurality of inductors arranged in series with each other, with the biasing power supply and with the trigger generating means, wherein the switch control inputs are connected to junctions located between respective pairs of the inductors so that in use: a negative bias voltage from the biasing power supply is applied to each of the switch control inputs, thereby to maintain the thyratrons in the OFF state in the absence of a trigger pulse; and when a trigger pulse is supplied by the input means, that trigger pulse is applied to all the switch control inputs via the inductors. More particularly, the thyratrons may each comprise a cathode, an anode and a grid disposed between the anode and the cathode, the grids serving as the switch control inputs and being coupled to the input to receive the triggering pulse. The apparatus may further provide for the heating of the cathodes of the thyratrons and for the deionising of the thyratrons after the discharge of the pulse forming networks by maintaining a proper relationship between the network impedances and the load resistance. Inadvertent discharge of the pulse forming networks back into the high voltage power supply can be prevented by the use of a charging diode. According to another aspect of the invention, there is provided a method of producing repetitive short high voltage pulses, the method comprising: resonantly charging a plurality of pulse forming networks by coupling said pulse forming networks in parallel with a high voltage power source; providing a trigger pulse to initiate switching of said pulse forming networks; discharging the plurality of pulse forming networks by applying the triggering pulse to close a plurality of thyratrons, arranged between the pulse forming networks, thus to connect the pulse forming networks in series with an external load; and repeating said charging, said providing of a trigger pulse and said discharging at a predetermined repetition frequency defined by said trigger pulses, characterised in that a negative biasing voltage is applied to respective control inputs of the thyratrons via a series of inductors to provide a triggering threshold to avoid inadvertent closure of the thyratrons during charging, the switch control inputs being connected to junctions located between respective pairs of the inductors, and in that, when initiating discharge, the trigger pulse is applied to the control inputs of all of the thyratrons via said series of inductors. For a better understanding of the invention and to show how the same may be carried into effect, reference will now be made, by way of example, to the accompanying drawing, wherein: Figure 1 is a schematic diagram of a modulator; and Figure 2 is a schematic diagram of a pulse forming network of the modulator of Fig. 1. The presently described embodiment is a modulator for producing repetitive short high voltage pulses having relatively fast rise and fall times and which does not require a transformer to step up the pulse voltage. Referring to Figs. 1 and 2, a schematic diagram of a modulator circuit 10 is illustrated. The modulator 10 has a power input terminal 18 which receives an alternating current (AC) prime power input, and a triggering input terminal 22 which receives an input triggering pulse. As will be fully described below, the triggering pulse initiates a high voltage output pulse which is provided to a load 70. The modulator 10 further includes a high voltage power supply 12, a trigger transformer 16, a cathode heater transformer 14, and a bias supply 34. The high voltage power supply 12 is a DC power supply which is connected to the input terminal 18, receives the AC prime power from terminal 18 and produces a charging voltage. The high voltage power supply 12 has a positive and a negative output terminal. The negative terminal is electrically connected to a charging inductor 24, which in turn is electrically connected to a diode 26. The positive terminal is electrically connected to ground. The cathode heater transformer 14 also receives the prime power from terminal 18 and produces cathode heater voltage. The trigger transformer 16 receives the trigger pulse from input terminal 22, and produces a trigger voltage. The bias supply 34 maintains a negative voltage to prevent inadvertent triggering of the modulator, as will be further explained below. The modulator 10 has a plurality of network/switch stages which are substantially identical. For purposes of explanation, only the first stage will be described, but it should be appreciated that the additional stages operate in substantially the same manner. Each stage comprises a pulse forming network 40, a thyratron 60, a first inductor 46, a second inductor 48, a third inductor 52, and a fourth inductor 54. As illustrated in Fig. 2, the pulse forming networks 40 have capacitors 72 and inductors 74 connected in a "ladder" circuit to form an artificial transmission line with a time delay equal to one half the desired pulse length. Pulse forming networks are well known to those skilled in the art, and other such equivalent networks could be substituted as desired. The pulse forming networks 40 have a positive and a negative terminal, with the positive terminal coupled to ground. The first inductor 46 1 is also electrically coupled to the positive terminal of the pulse forming network 40, and in series with each of the first inductors 46 2 , 46 3 and 46 4 , which electrically connect each of the positive terminals of the pulse forming networks 40 together and to ground. The thyratrons 60 have a cathode 62, an anode 64, a grid 66 and a cathode heater 68. The thyratrons are filled with a hydrogen gas which ionizes when a proper trigger signal is applied between the grid 66 and the anode 64, resulting in high current flow between the cathode 62 and the anode. The negative voltage applied by the bias supply 34 to the grid 66 acts as a gate for electrons and maintains the thyratrons in the off state until the trigger pulse is applied. The potential across the cathode heater 68 maintains the cathode surface at a temperature which permits thermionic emission of electrons from the cathode surface. The cathodes 62 are electrically connected to the negative terminal of the pulse forming networks 40. The second inductor 48 1 electrically connects the cathode 62 of thyratron 60 1 and negative terminal of the pulse forming network 40 1 to the high voltage power supply 12 through the diode 26, and is electrically coupled in series to each of the second inductors 48 2 , 48 3 , and 48 4 , which electrically connect to each of the cathodes 62 and pulse forming networks 40 of the stages. The third inductor 52 1 electrically connects the cathode heater 68 of the thyratron 60 1 to the heater transformer 14, and is electrically coupled in series to each of the third inductors 52 2 , 52 3 and 52 4 , which electrically connect to each of the cathode heaters 68. The fourth inductor 54 1 electrically connects the triggering transformer 16 with the grid 66 of the thyratron 60 1 , and is electrically coupled in series to each of the fourth inductors 54 2 , 54 3 and 54 4 , which electrically connects to each of the grids 66. The inductors 48, 52 and 54 are wound in trifilar configuration so that the same voltage will exist across the inductors due to the large mutual inductance and capacitance between the windings. In operation, the inductors 48 and 46 connect the pulse forming networks 40 in parallel with the high voltage power supply 12 at low frequencies comparable to the repetition frequency of the modulator, charging the pulse forming networks by resonant charging to a voltage level equal to twice the voltage of the high voltage power supply 12. The charging inductor 24 has a much larger inductance than that of any of the other inductors in the circuit, such that it is resonant at slightly more than half the maximum repetition frequency when combined with the capacitances of all the pulse forming networks 40 in parallel. When prime power is furnished to the high voltage power supply 12, current flows through the charging diode 26 and the charging inductor 24, charging all the pulse forming networks 40 to twice the power supply voltage in a sinusoidally varying waveform. The diode 26 prevents the discharge of the pulse forming networks 40 back into the high voltage power supply 12. When a triggering voltage provided at input 22 overcomes the negative bias on the thyratrons 60 due to the bias supply 34, current flows from the cathodes 62 to the anodes 64 of all the thyratrons simultaneously, connecting all the pulse forming networks 40 in series. Because each pulse forming network 40 delivers a voltage nearly equal to one-half the voltage to which it is charged, the four networks illustrated in Fig. 1 will produce a pulse very nearly equal to four times the power supply voltage if the load impedance is slightly less than four times the pulse forming network impedance. In one example, the impedance of the pulse forming networks 40 is slightly higher than its share of the load impedance so the voltage on the pulse forming network will reverse slightly after the pulse and deionize the thyratrons 60. The diode 78 and resistor 76 in Fig. 2 will dissipate this reverse voltage and prepare the network for the next charging cycle. With a high voltage power supply 12 having a voltage output of 10 kilovolts, the voltage of the pulse provided to the load 70 would be four times the power supply voltage, or 40 kilovolts. It should be apparent to those skilled in the art that higher voltage pulse values can be obtained by adding additional stages beyond the four stages illustrated in Fig. 1. It should also be apparent that the components of the modulator 10 may be configured to provide positive going voltage pulses as well as the negative going pulses described above.

Claims: An apparatus for producing repetitive voltage pulses comprising: a high voltage power supply (12) for producing a charging voltage; a plurality of pulse forming networks (40 1 ...40 4 ) for storing charge; a plurality of thyratrons (60 1 ...60 4 ), arranged between respective ones of said pulse forming networks and an external load (70), the thyratrons being switchable via respective switch control inputs (66) between an OFF state in which the pulse forming networks are connected in parallel with the high voltage power supply (12), for resonantly charging the pulse forming networks, and an ON state in which the thyratrons provide a current path connecting the pulse forming networks in series, for discharging the pulse forming networks into an external load (70); and input means (16) to receive a trigger pulse and coupled to the thyratrons so that receipt of a trigger pulse by the input means (16) will switch the thyratrons (60 1 ...60 4 ) into the ON state; characterised by a biasing power supply (34) for negatively biasing the thyratrons (60 1 ...60 4 ) and a plurality of inductors (54 1 ...54 4 ) arranged in series with each other, with the biasing power supply (34) and with the trigger generating means (16), wherein the switch control inputs (66) are connected to junctions located between respective pairs of the inductors (54) so that in use: a negative bias voltage from the biasing power supply (34) is applied to each of the switch control inputs (66), thereby to maintain the thyratrons (60 1 ...60 4 ) in the OFF state in the absence of a trigger pulse; and when a trigger pulse is generated by the input means (16), that trigger pulse is applied to all the switch control inputs (66) via the inductors. An apparatus according to claim 1 arranged so that in use the pulse forming networks are resonantly charged to a potential equal to at least about twice the voltage of said high voltage power supply (12). An apparatus according to claim 1 arranged so that in use the pulse forming networks are resonantly charged to a potential equal to substantially twice the voltage of said high voltage power supply (12). An apparatus according to claim 1, 2 or 3 and comprising charging inductor means (24) arranged to provide a current path from the high voltage power supply (12) to the pulse forming networks during resonant charging. An apparatus according to claim 4, wherein the charging inductor means (24) has an inductance greater than that of inductances coupling the pulse forming networks, the inductances coupling the pulse forming networks comprising a second plurality of inductances (48 1 ...48 4 ) arranged in series with each other, one end of the series being connected to the charging inductor means and the inductances of the second plurality being connected to respective ones of the pulse forming networks, wherein the inductors of the first-mentioned plurality of inductors are wound together with respective inductors of the second plurality to maintain the same voltage across each wound pair. An apparatus according to claim 4 or 5 and comprising diode means (26) arranged in reverse orientation in said current path, so as to prevent current flow back into the high voltage power supply (12) during discharge of the pulse forming networks. An apparatus according to any one of the preceding claims, wherein the impedance of the pulse forming networks is such that in use, after discharge of the pulse forming networks, the voltage on the pulse forming networks will reverse so as to deionize the thyratrons. An apparatus according to any one of the preceding claims, wherein the input means comprises a transformer (16) one side of which provides the trigger pulse input (22) and the other side of which is connected between the bias power supply (34) and a first one of the inductors. An apparatus according to any preceding claim, wherein each of the thyratrons comprises a cathode (62), an anode (64) and a grid (66) disposed between said anode and said cathode, the thyratron grids serving as said switch control inputs. An apparatus according to claim 9, wherein the biasing power supply (34) is arranged to negatively bias the grid (66) of each thyratron with respect to its cathode (62). An apparatus according to claim 9 or 10 and comprising means (68) for heating the cathodes of the thyratrons. An apparatus according to claim 9, 10 or 11, when appended to claim 5, and comprising: a further plurality of further inductors (46 1 ...46 4 ), each of said further inductors coupling a first terminal of said pulse forming networks (40) together and to ground; whereby the inductor of said second plurality of inductors couple second terminal of said pulse forming networks together and to said charging inductor means (24) ; and wherein the thyratron cathodes (62) are coupled to said second terminal of said pulse forming networks and the thyratron anodes (64) are coupled to said first inductors, so that in use the pulse forming networks are charged in parallel by the high voltage power supply (12) and via said further inductor and said inductances of the second plurality when non-conductance between the associated cathodes and anodes provides said OFF state. An apparatus according to claim 12 and comprising a plurality of third inductors (52 1 ...52 4 ) arranged to conduct heater current to respective heaters (68) of the thyratrons. An apparatus according to claim 13, wherein said inductances of the second plurality and said third inductors and the first-mentioned inductors are wound in trifilar configuration. An apparatus according to any one of the preceding claims, wherein the thyratrons are arranged for high voltage operation so that the high voltage power supply (12) is able to deliver a voltage to an external load (70) of at least of the order of tens of kilovolts. A method of producing repetitive short high voltage pulses, the method comprising: resonantly charging a plurality of pulse forming networks by coupling said pulse forming networks in parallel with a high voltage power source (12); providing a trigger pulse to initiate switching of said pulse forming networks; discharging the plurality of pulse forming networks by applying the triggering pulse to close a plurality of thyratrons, arranged between the pulse forming networks, thus to connect the pulse forming networks in series with an external load (70); and repeating said charging, said providing a trigger pulse and said discharging at a predetermined repetition frequency defined by that of the trigger pulse, characterised in that a negative biasing voltage is applied to respective control inputs (66) of the thyratrons via a series of inductors to provide a triggering threshold to avoid inadvertent closure of the thyratrons during charging, the switch control inputs (66) being connected to junctions located between respective pairs of the inductors, and in that , when initiating discharge, the trigger pulse is applied to the control inputs (66) of all of the thyratrons via said series of inductors. A method according to claim 16, said pulse forming networks being charged to a potential equal to at least about twice the voltage of said high voltage power supply (12). A method according to claim 16, said pulse forming networks being charged to a potential equal to substantially twice the voltage of said high voltage power source (12). A method according to claim 16, 17, or 18, wherein, during said discharging, the pulse forming networks are prevented from discharging into said high voltage power source. A method according to claim 16, 17, 18 or 19, wherein, in each repetition, the thyratrons are deionized by a voltage reversal after discharge of said plurality of pulse forming networks.

Inventors: SYMONS ROBERT SPENCER, SYMONS, ROBERT SPENCER
Applicants: LITTON SYSTEMS INC, LITTON SYSTEMS, INC.
Application year: 1994
Main classification: H03K3
IPC codes: H03K3